This invention relates to articles made of alpha-beta titanium alloys and, more particularly, to a processing technique used to obtain optimized properties in different regions of the articles.
Properly processed titanium alloys exhibit good properties at room-to-intermediate temperatures, and are of low density as compared with steel, nickel, and cobalt alloys. Titanium alloys are used in aircraft gas turbine (jet) engines in components that are exposed to intermediate temperatures during service. For example, heat-treated and/or thermomechanically processed titanium alloys are used in rotating components such as fan disks, fan blisks, high-pressure compressor disks, and high-pressure compressor blisks that operate at temperatures as high as about 600.degree. C. during service.
Rotating components such as disks and blisks have material performance requirements that vary according to the location on the article. A blisk is a disk with integral blades extending from the outer periphery of the disk region. The disk region may be either solid or annular with a bore therethrough. The central region of the disk requires good crack growth properties and good fracture toughness. The airfoil regions of the blades require good fatigue properties and ductility to resist foreign object damage.
Alpha-beta (including near-beta) titanium alloys are currently used in a number of disk and blisk applications. Such alloys have equilibrium phase diagrams with an equilibrium beta phase stable at temperatures above about 850-1050.degree. C. At much lower temperatures, the alpha phase maybe thermodynamically stable, but because of kinetics considerations a mixture of alpha and beta phases is usually observed. Some alloys may exhibit nearly 100 percent alpha phase at lower temperatures, although alloy chemistry balance and kinetic considerations generally preclude this. However, the equilibrium phase diagrams provide guidelines as to the nature of the phases typically present in the alloys. The well-known Ti-6A1-4V alloy is an example of an alpha-beta titanium alloy, and the Ti-6A1-2Sn-4Zr-6Mo alloy is an example of a near-beta titanium alloy that is within the scope of the "alpha-beta" titanium alloys as used herein.
The alpha-beta titanium alloys may be thermally or thermomechanically processed to produce various types of useful properties. For example, processing in the beta phase field typically leads to an alloy with good fracture toughness, crack growth, and creep properties, but less-than-optimal fatigue properties. Similarly, processing in the alpha-plus-beta range leads to good ductility and fatigue properties but less-than-optimal fracture toughness.
Thus, the available alpha-plus-beta titanium alloys do not provide a combination of properties that is optimized for performance in both the central and blade regions of a blisk, or in the central and rim regions of a disk. There have been many attempts, with varying degrees of success, to develop improved alloys and to identify optimized heat-treatment approaches that lead to an improved combination of properties for use in such disks and blisks. However, there remains a need for an improved approach to the manufacture of titanium-alloy articles for use in applications such as the rotating components of aircraft gas turbine engines. The present invention provides such an improved approach, and further provides related advantages.